Cloud storage is a model of networked online storage where data are stored on multiple virtual servers, usually hosted by third parties (e.g., parties who are not owners of the stored data), rather than being hosted on dedicated servers. These third-party hosting companies operate large data centers, and in the background, virtualize the resources according to the requirements of their customers (e.g., the owners of the stored data) and expose the resources as storage pools, which the customers can themselves use to store files or data objects. Physically, the resources may span across multiple servers.
As a part of data security, the data stored in a cloud storage, especially in a public cloud storage, are sometimes encrypted. A correct key is required to decrypt the encrypted data before the data can be accessed by anyone. In addition, the data are often duplicated into multiple copies and distributedly stored on multiple servers so that if one copy of the data is unavailable due to any reason (e.g., device failure), there is at least another copy of the same data that can be accessed.